A conventional control for distributing brake forces between the front and rear wheels is, for example, described in Japanese Patent Publication No. 50-9949, which claims the priority of U.S. patent application Ser. No. 532,471 filed Mar. 7, 1966, now U.S. Pat. No. 3,423,936. The distribution control is carried out by a brake fluid pressure control valve. The control valve comprises a housing and a piston positioned in the housing. The housing includes a stepped cylindrical chamber with a large diameter portion and a small diameter portion communicating with the larger diameter portion. A master cylinder port and a rear wheel cylinder port are formed on the housing. Both the ports communicate with the large and small diameter portions of the cylindrical chamber, respectively.
A seal made of flexible material is disposed adjacent to the small diameter portion between the piston and the large diameter portion. The seal comprises an annular base portion and an annular lip portion. A plurality of radial ribs are formed on a first end surface of the base portion. The ribs have a semi-circular section. The base portion also has a plurality of axial ribs of semi-circular section provided on its outer peripheral surface. The seal is adapted to be placed at a prescribed position in the large diameter portion of the cylindrical chamber, namely, at a position where the ribs of the base portion contact the internal wall of the large diameter portion as well as an end wall or a shoulder between the large diameter portion and the small diameter portion. The annular base portion defines a valve seat at its inner perimeter adjacent to the small diameter portion of the cylinder chamber. The lip portion touches the internal wall of the large diameter portion of the chamber. The lip permits the brake fluid to pass between an outer perimeter and the first end surface of the base portion and the internal wall and end wall of the large diameter portion, thereby flowing from the small diameter portion to the large diameter portion. However, the lip portion prevents the reverse flow of the fluid.
The piston passes through the inner perimeter of the base portion of the seal with a clearance left therebetween. The piston bears the fluid pressure of the rear wheel cylinders at a first end positioned adjacent to the seal. The opposite end thereof (i.e., the second end) is slidably inserted in a sealed space. The piston has a valve head and a shoulder. The valve head moves back and forth relative to the seat between a seated position and an unseated position in accordance with the fluid pressure. Likewise, the step alternately contacts and is separated from a plurality of protrusions formed on the second end surface of the seal. A spring biases the valve head away from the valve seat.
When the brake fluid pressure control valve as constructed above is operated, the valve head of the piston is normally held away from the valve seat at a predetermined position. When the fluid pressure in the rear wheel cylinder reaches a predetermined level, the fluid pressure in the rear wheel cylinder is equal to the fluid pressure in the master cylinder. When the fluid pressure in the rear wheel cylinder exceeds the predetermined level, the piston is displaced such that the valve head sits on the valve seat. Then, the piston reciprocates in accordance with an increase in the fluid pressure in the master cylinder, whereby the valve head repeatedly sits on and separates from the valve seat. As a result, the fluid pressure in the rear wheel cylinder is controlled in accordance with the following equation. EQU PW=(1-SS/SL)PM+F/SL
where
PW: Fluid pressure in the rear wheel cylinder PA0 PM: Fluid pressure in the master cylinder PA0 SS: Sectional area of the second end of the piston PA0 SL: Area of a circle having a diameter equal to a sealing diameter defined by contact of the valve head with the valve seat PA0 F: Load of the spring when the valve head sits on the valve seat
In the above described device, the seal is held in a fixed position in the cylinder chamber. However, when the valve head sits on the valve seat, the flexible seal has its protrusions separated from the step of the piston. In this position, the axial movement of the seal toward the small diameter portion is regulated by the engagement of the radial ribs with the shoulder of the cylinder chamber. However, the seal has its axial movement toward the large diameter portion prevented only by a portion of the annular lip corresponding to its interference. That is, the free end of the lip that contacts the internal wall of the large diameter portion holds the seal in order to retain it at the fixed position in the large diameter portion of the cylinder chamber. Thus, the seal is in a floating state. In other words, the flexible seal is in an unstable state in which it may move dependent on the fluid pressure differential between the rear wheel cylinder and the master cylinder. Therefore, anytime there is a rapid change (rapid increase or decrease) in the fluid pressure in the master cylinder, the seal may move in an axial direction away from the fixed position. In this case, there is no problem if the seal moves axially in accordance with the change of the fluid pressure. However, since the seal is in the unstable state as mentioned above, it cannot accurately follow the change of the pressure difference nor move smoothly in the axial direction. Therefor, it is possible that the seal will be slanted relative to the axis of the cylinder and therefore, unable to fully perform its function. As a result, the fluid pressure in the system will be unevenly divided.
This problem may be solved by providing sufficient interference to the axial ribs of the seal. However, if the axial ribs are not molded to the same size, the dimensions of the valve seat vary which will again cause uneven pressure distribution.